A solution of a rigid polymer, the so-called liquid-crystalline polymer, allows preparation of a highly oriented molding having high strength and a high elastic modulus due to such properties that molecular chains are readily oriented in the direction of flow and a long time is required to randomly direct the molecular chains once the same are oriented. Further, the rigid polymer can provide a heat-resistant molding due to a high glass transition temperature.
A rigid polymer, particularly polybenzazole polymer dissolves only in limited mineral acids such as methanesulfonic acid and polyphosphoric acid. Such a polymer is generally molded from either a low-concentration mineral acid solution of not more than 2% or a high-concentration mineral acid solution of at least 14% such as that described in Example of Japanese Patent Laying-Open No. 63-210138. When worked into fiber or a film of a simple body of polybenzazole, the polymer is molded from a high-concentration liquid crystal dope described in U.S. Pat. No. 5,552,221 or U.S. Pat. No. 5,367,042. When a composite containing a polymer network of polybenzazole as a reinforcer is prepared from such a high-concentration liquid crystal dope, open space in the polymer network are so reduced that it is difficult to homogeneously impregnate other components. When the polymer network is prepared from a low-concentration solution of not more than several percent, it follows that a large quantity of mineral acid is used for manufacturing a product, unsuitably for an industrial process. In order to prepare a composite containing a polymer network of a heat-resistant rigid polymer as a reinforcer, therefore, a technique absolutely different from that for a conventional rigid polymer molding is necessary.
As to a rigid polymer composite, a method of molding the same from a starting material of a blend or a copolymer with a flexible polymer such as polyamide or polyetherketone has been studied. With this method, however, no high-performance material making the best use of the characteristics of the rigid polymer has not yet been put into practice under the present circumstances.
On the other hand, Japanese Patent No. 2,522,819 describes an invention related to a composite prepared by previously molding a continuous porous network body of an aromatic heterocyclic polymer and impregnating the same with binder resin. With this method, however, it is so difficult to obtain a homogeneous continuous porous network body having large voids that no composite material having satisfactory performance can be obtained.
The present invention relates to a technique for preparing a high-performance composite material by molding and coagulating a solution of a rigid polymer for forming a polymer network and impregnating voids thereof with a functional polymer or an inorganic matrix excellent in compressive strength.